The present invention relates to a mold frame for an apparatus for producing shaped concrete blocks from a mold, more particularly, to such a mold frame which consists of longitudinal and transverse frame members which are connected to each other and which surround a mold insert which contains several mold cavities.
The German patent DE 195 08 152 A1 discloses a mold frame in which two longitudinal frame parts and two transverse frame parts are welded together at their ends to form the four corners of a mold frame. The outer faces of the transverse frame parts have a flange which provides a connection to a lifting frame of the molding machine. The inner faces of the mold frame are constructed as oblique surfaces in which is retained the mold insert. The outer surfaces of the mold insert have triangular prisms which contact the oblique surfaces of the mold frame when the mold insert is inserted into the welded mold frame. In order to retain the mold insert in the mold frame pressure strips are threaded from underneath onto the mold frame and adjoin with one oblique surface the bottom of the triangular prisms of the mold insert. Resilient or elastic damping pads or plates are inserted between the oblique surfaces of the mold frame and the mold insert.
During the vibration process, the mold frame with the mold insert rests upon the board of the vibrating table and is held and pressed down by the molding machine. As a result, the mold frame is subjected to high alternating stresses which often produce cracks in the welds of the frame parts and also frame factures as a result of the stresses which occur during welding. In addition, the threaded connection between the mold frame and the mold insert can become loosened or fractures may occur since the danger of fatigue fracture is increased because of the blind hole in which the screw is received and also because of the relatively short length of these mounting screws.
Generally in conjunction with a welded mold frame, weld connections are provided between the mold frame and the hardened mold insert. The resulting notch stresses lead to a significant reduction of service or operating life of the mold frame. In addition, merely providing these welded connections greatly increases the production and assembly costs of the molding operation.
It has also been proposed that the frame parts of the mold frame be threadedly connected to each other. However, this proposal has the disadvantage that as a result of shaking vibrations and the resulting torques which are transmitted only by the forces of friction within the threaded connections, a sufficient and permanent stability of the mold frame cannot be achieved. Because of the limited space conditions and the required large dimensions of the mounting screws only a single screw or threaded connection can be used at each frame corner. The torques which occur due to vibrations in the frame corners cannot be accommodated merely by the pretensioning forces of the screw and relative movements occur between the components secured by the threaded connection.